Although prostate and breast cancers account for significant morbidity and mortality, relatively little is known about the molecular mechanisms involved in the pathogenesis of these diseases. In order to provide in vivo systems in which such processes can be studied in great detail, genetic constructs have been introduced into the germ line of mice in an attempt to produce transgenic models of prostate and breast cancers. Such animal model systems will be extremely valuable in furthering the understanding of the relationships between oncogenes, growth factors, and receptors to the development of prostate and breast cancers. In addition, such models will be very useful for devising therapies to treat these malignancies. Prostate- and breast-specific expression vectors may also be of great value for use in gene therapy-related treatment of these diseases. Numerous transgenic animals have been made using a hormone- responsive promoter driving the transcription of a powerful oncogene. Several transgenic animals have developed prostate hyperplasia, breast cancer, osteosarcoma, and follicular thyroid carcinoma. Work is ongoing to improve the specificity of the expression of the transgene to the prostate and breast.